POS0060 MEASUREMENTS OF COMPLEMENT ACTIVITY SPECIFICALLY THOUGH THE LECTIN -OR THE CLASSICAL PATHWAY IN PATIENTS WITH SLE
Authors: M. C. L. Larsen, A. Troldborg, E. Toonen, L. Hurler, Z. Prohászka, L. Cervenak, A. G. Hansen, S. Thiel
Journal: Annals of the Rheumatic Diseases (2022)

Background In systemic lupus erythematosus (SLE), the complement system (CS) is activated and thought to occur primarily through the classical pathway (CP) [1]. Traditionally, when diagnosing SLE or assessing disease activity, measurement of low C3 or C4 are used as proxies for complement activation [2]. However, measurement of C3 and C4 does not differentiate which complement pathway initiated the activation (i.e., the lectin pathway (LP), the CP, or the alternative pathway) [1, 3]. C1-esterase inhibitor (C1inh) is one of the key regulators of the CS. C1inh is the exclusive inhibitor of the active CP enzymes C1r and C1s [4], and the major inhibitor of active LP enzymes MASP-1 and MASP-2 [5]. A possible way of assessing complement activation through a specific pathway, is by measuring activated enzymes complexed with C1inh in plasma, as these complexes only exist after complement enzyme activation. Objectives Our aim was to investigate and unravel LP and CP complement activation in SLE, by measuring the protein complexes MASP1/C1inh (LP specific activation) and C1r/C1inh (CP specific activation). Furthermore, we aimed to investigate whether there is an association between complement activity, disease activity (SLEDAI) and disease manifestations (lupus nephritis (LN)). Methods A cross sectional cohort of 150 patients with SLE fulfilling the 1997 ACR classification criteria for SLE were included from the out-patient clinic at the department of Rheumatology, Aarhus University Hospital (AUH), Denmark. Disease manifestations and disease activity using SLEDAI score was assessed at inclusion. Fifty healthy individuals included at the Blood Bank, AUH, were used as controls. Both C1s/C1inh and MASP1/C1inh complexes were measured in all samples using two newly developed sandwich ELISAs (C1s/C1inh: cat# HK399; MASP1/C1inh: Cat#3001, Hycult Biotech, Uden, The Netherlands). EDTA-samples from both SLE patients and controls were measured in duplicates. Results When comparing SLE patients to controls, we observed a difference in complement activation through the LP, where a lower mean MASP1/C1inh plasma concentration was observed (p&lt;0.01). C1s/C1inh concentrations were significantly increased in active SLE patients (SLEDAI &gt;6) when compared to SLE patients with low disease activity (SLEDAI &lt;6, p&lt;0.01) and correlated with SLEDAI score (r=0.285, p&lt;0.01). C1s/C1inh concentrations were increased in SLE patients with active LN compared to non-active LN, however this not statistically significant (p=0.09). No differences in MASP1/C1inh plasma concentrations were observed between active SLE patients and patients with low disease activity (p=0.11), nor did we observe a significant correlation with disease activity (r=0.12, p=0.13). In active LN, plasma concentrations of MASP1/C1inh were significantly elevated compared to non-active LN (p=0.02). Conclusion Our data suggest that the CP and the LP is activated in SLE. CS is generally activated in active SLE disease, whereas activation of the LP might be more specific to particular disease manifestations like LN. Our findings warrant further research into activation of the specific complement pathways in relation to specific disease manifestations in SLE. References [1]Troldborg, A., et al., J Rheumatol, 2018. [2]Weinstein, A., et al., Curr Rheumatol Rep, 2021. [3]Atkinson, J.P., Clin Exp Rheumatol, 1989. [4]Sim, R.B., et al., FEBS Lett, 1979. [5]Paréj, K., et al., Mol Immunol, 2013. Figure 1. Measurements of plasma concentrations of C1r in complex with C1-inhibitor (C1r/C1inh) (a1-a4) and MASP1 in complex with C1-inhibitor (MASP1/C1inh) (b1-b4) in Systemic Lupus Erythematosus (SLE) patients and controls. A1 and b1 show measurements of SLE patients versus controls. A2 and b2 show measurements in SLE patients with active disease (SLEDAI &gt;6) versus low disease activity (SLEDAI&lt;6). A3 and b3 show correlation analysis between measurements and SLEDAI score in SLE patients. A4 and b4 show measurements in SLE patients with active lupus nephritis (LN) versus non-active LN. Disclosure of Interests Mads Christian Lamm Larsen: None declared, Anne Troldborg: None declared, Erik Toonen Employee of: ET is an employee of Hycult Biotech, Lisa Hurler: None declared, Zoltán Prohászka: None declared, László Cervenak: None declared, Annette G Hansen: None declared, Steffen Thiel: None declared